The present invention relates to a method for sealing a porous aluminum oxide film formed on an aluminum substrate, and more particularly, to a method for improving the corrosion resistance and organic coating adhesion characteristics of an aluminum oxide film produced by anodization.
Aluminum alloys used in aircraft and other lightweight structures are highly susceptible to corrosion, especially in a saline environment. A common way to prepare aluminum alloys to protect them from corrosion is to form an aluminum oxide layer on the surface of the alloy by anodizing the alloy in the presence of an acid. For example, after an aluminum alloy part has been cleaned, rinsed and deoxidized, the metal part is anodized in an electrolyte solution containing on the order of 5% by weight of chromic acid. The anodization is carried out at potentials ranging from 22 to 60 volts for periods on the order of 30 to 60 minutes at temperatures around 95.degree. F. The aluminum oxide thus formed still does not provide good resistance to corrosion because it is very porous. Thus, the aluminum oxide layer must be sealed to prevent corrosive media such as sodium chloride from penetrating through the aluminum oxide layer to the base metal.
A variety of methods for sealing aluminum oxide layers so formed have been developed. Examples of some of the prior methods include contacting the aluminum oxide layer with a hot sealing bath comprising deionized water, an aqueous solution of dichromate ions, nickel acetate, a strong aqueous solution of chromic acid and sodium tungstate, or a dilute aqueous solution of chromic acid. These sealing methods involve at least in part the conversion of the outer surface of the aluminum oxide layer to aluminum oxide monohydrate. Although the mechanism is not completely understood, the resulting porous surface of the aluminum oxide is sufficiently sealed to prevent the incursion of corrosive media. While the foregoing methods do accomplish the objectives of sealing the aluminum oxide layer, most are difficult to control, and thus achieve varying degrees of corrosion resistance. In addition, many times it is difficult to obtain good adherence of an organic coating, such as an epoxy paint or primer, to a sealed aluminum oxide surface that effectively resists corrosion in a saline environment.
For example, when a hot deionized water sealing bath is employed, it is difficult to control the degree of hydration of the oxide and thus the degree of corrosion protection provided by the sealed anodized layer. Satisfactory corrosion resistance in a saline environment can be achieved by increasing the amount of hydration to on the order of 15 percent or more, thus sealing the porous oxide. However, this extent of hydration forms a cohesively weak oxide, and thus significantly degrades adhesion of organic coatings to be sealed surface. The chromic acid/chromium tungstate sealing process was developed primarily for aluminum oxide layers that are anodized in the presence of a sulfuric acid bath to provide a colorless coating. Additionally, the process is conducted at a relatively high pH, on the order of 6.0, which tends to form a cohesively weak oxide. Although sealing in the presence of a dilute chromic acid solution having a relatively low pH tends to provide a surface to which organic coatings will adhere at least to a certain extent, the process has not met with great success because the corrosion resistance of the product in a saline environment is generally not satisfactory. Sealing in the presence of a 5% by weight sodium or potassium dichromate has been considered a viable alternative to sealing in the presence of deionized water, the method currently being used. However, sealing in the presence of dichromate ion requires a separate rinsing operation after the sealing step. This additional step greatly increases the expense of forming the corrosion resistant coating. Moreover, it has been found that sealing in the presence of dichromate ion does not provide organic coating adhesion as good as desired.